1. Technical Field
The invention concerns a synchronous digital communications system, a control installation for a network node of a synchronous digital communications system, a network element and a central clock generator for a network node of a synchronous digital communications system.
2. Discussion of Related Art
A synchronous digital communications system is for example an SDH (Synchronous Digital Hierarchy) system or an SONET (Synchronous Optical Network) system. When messages are transmitted via such a communications system, it is essentially necessary that all the network elements (e.g. Add/Drop-Multiplexer, Crossconnects or Line Multiplexer) operate synchronously with each other. ITU recommendation G.803 also states that a network node containing several network elements must be supplied with a reference clock by a single central clock generator of the network node.
A conference article by Dr. M. Wolf, 8th. European Frequency and Time Forum 9., Nov. 3, 1994, TU Munich, pages 166 to 174, describes how the synchronization of the network elements can take place in such a synchronous digital communications system: One network element receives message signals at two inputs, from which a clock is derived and can be used for the synchronization. The accuracy of the clocks in these message signals is provided by a contained quality indicator, hereafter called SSM (Synchronization Status Message, see e.g. ITU-T recommendations G.707, G.708 or G.709). By means of the SSM a selection device in the network element selects one of the clocks as the reference clock and routes it to a central clock generator of the network node. This central clock generator distributes the reference clock to all the network elements of the network node. It also describes that the central clock generator can be an independent unit, or that the clock generator of a network element, preferably that of a crossconnect, can be used as a central clock generator. To prevent the formation of synchronization loops, i.e. where two network elements use each other as mutual reference clock sources, an additional SSM is defined: "Do Not Use for synchronization", hereafter called DNU. It is transmitted from a network element to all the outputs that are connected to the network element which was selected as the reference clock source.
One problem which is linked to the represented solution is that the network elements of the network node do not know the origin of the selected reference clock, or its accuracy. Nor can the central clock generator provide this information because the clock is selected by the selection device of a network element. As the accuracy of their reference clock, the network elements can only indicate in the SSM the accuracy of the central clock generator during open unsynchronized operation, which as a rule corresponds to the accuracy established in the ITU-T G.812. Also, the SSM DNU cannot be transmitted to the outputs which are connected to the reference clock source used for the synchronization, since this reference clock source is not known to the network elements. This can lead to the formation of synchronization loops.